From polar night to midnight sun: Diel vertical migration, metabolism and biogeochemical role of zooplankton in a high Arctic fjord (Kongsfjorden, Svalbard)

Source at http://dx.doi.org/10.1002/lno.10519 Zooplankton vertical migration enhances the efficiency of the ocean biological pump by translocating carbon (C) and nitrogen (N) below the mixed layer through respiration and excretion at depth. We measured C and N active transport due to diel vertical migration (DVM) in a Svalbard fjord at 79°N. Multifrequency analysis of backscatter data from an Acoustic Zooplankton Fish Profiler moored from January to September 2014, combined with plankton net data, showed that Thysanoessa spp. euphausiids made up > 90% of the diel migrant biomass. Classical synchronous DVM occurred before and after the phytoplankton bloom, leading to a mismatch with intensive primary production during the midnight sun. Zooplankton DVM resulted in C respiration of 0.9 g m−2 and ammonium excretion of 0.18 g N m−2 below 82 m depth between February and April, and 0.2 g C m−2 and 0.04 g N m−2 from 11 August to 9 September, representing > 25% and > 33% of sinking flux of particulate organic carbon and nitrogen, respectively. Such contribution of DVM active transport to the biological pump in this high-Arctic location is consistent with previous measurements in several equatorial to subarctic oceanic systems of the World Ocean. Climate warming is expected to result in tighter coupling between DVM and bloom periods, stronger stratification of the Barents Sea, and northward advection of boreal euphausiids. This may increase the role of DVM in the functioning of the biological pump on the Atlantic side of the Arctic Ocean, particularly where euphausiids are or will be prevalent in the zooplankton community

Understanding winter patterns of zooplankton Diel Vertical Migration (DVM) in a high Arctic fjord (Kongsfjorden, Svalbard)

Recent Arctic studies contradict a long-held paradigm of winter quiescence by documenting activity at many trophic levels in the marine food web even during the darkest months of winter. Previous studies have failed to identify which species are performing patterns of vertical migrations during polar night in Kongsfjorden (Svalbard). In the present study, acoustic data supplemented by zooplankton net sampling demonstrated that polar night migration involved several migration patterns. Synchronized diel vertical migration (DVM) was present during the first and final parts of the polar night period, when day-night cycles were more distinct. During the darkest months of mid-winter, synchronized DVM was absent. Mid-winter migration patterns were restricted to surface waters, of unsynchronized character, and did not follow a clear diurnal pattern. Periodically, mid-winter migrations became more synchronized possibly due to enhanced light sources other than solar, such as lunar light. Copepods, chaetognaths and krill (Thysanoessa spp.) were abundant species but krill were responsible for the acoustic migration patterns. Spectral sensitivity tests showed that krill (T. inermis) were able to perceive solar background illumination during mid-winter. A clock gene investigated in krill required light to show circadian rhythmicity. The molecular and electrophysiological experiments suggested that polar night migration was exogenous governed by ambient light levels rather than endogenously controlled, although, light intensity had to be at a certain threshold to show diurnal rhythms. Light levels were probably too low during mid-winter, which explains unsynchronized migration patterns and lack of clock gene rhythms. Predation pressure was not evaluated in the present study but it may be that the ultimate driver for synchronized migration patterns was an anti-predatory response. This study presents a very first glimpse into a biosphere in which the dominant light source appears to trigger a response in zooplankton species, which does not always follow a diurnal pattern

Is ambient light during the high Arctic polar night sufficient to act as a visual cue for zooplankton?

The light regime is an ecologically important factor in pelagic habitats, influencing a range of biological processes. However, the availability and importance of light to these processes in high Arctic zooplankton communities during periods of 'complete' darkness (polar night) are poorly studied. Here we characterized the ambient light regime throughout the diel cycle during the high Arctic polar night, and ask whether visual systems of Arctic zooplankton can detect the low levels of irradiance available at this time. To this end, light measurements with a purpose-built irradiance sensor and coupled all-sky digital photographs were used to characterize diel skylight irradiance patterns over 24 hours at 79°N in January 2014 and 2015. Subsequent skylight spectral irradiance and in-water optical property measurements were used to model the underwater light field as a function of depth, which was then weighted by the electrophysiologically determined visual spectral sensitivity of a dominant high Arctic zooplankter, Thysanoessa inermis. Irradiance in air ranged between 1–1.5 x 10-5 μmol photons m-2 s-1 (400–700 nm) in clear weather conditions at noon and with the moon below the horizon, hence values reflect only solar illumination. Radiative transfer modelling generated underwater light fields with peak transmission at blue-green wavelengths, with a 465 nm transmission maximum in shallow water shifting to 485 nm with depth. To the eye of a zooplankter, light from the surface to 75 m exhibits a maximum at 485 nm, with longer wavelengths (>600 nm) being of little visual significance. Our data are the first quantitative characterisation, including absolute intensities, spectral composition and photoperiod of biologically relevant solar ambient light in the high Arctic during the polar night, and indicate that some species of Arctic zooplankton are able to detect and utilize ambient light down to 20–30m depth during the Arctic polar night